The invention relates to embodiments of ion cyclotron resonance cells, of which the ends are covered by electrode structure elements carrying electrostatic voltages of alternating polarity, and it relates to a method for excitation and detection of ions. In ion cyclotron resonance mass spectrometers (ICR-MS) the mass-to-charge ratios m/z of ions are measured using their orbiting motions in a homogeneous magnetic field of high field strength. The orbiting motion can consist of a superposition of cyclotron and magnetron motions. The magnetic field is usually generated by superconducting magnet coils, which are cooled by liquid helium. Currently these magnets offer a useful cell diameter between 6 and 12 centimeters at magnetic fields of 7 to 15 Tesla.
The ion's orbiting frequency is measured in ICR measurement cells, which are located within the homogeneous parts of the magnetic field. The ICR measurement cells usually consist of four longitudinal electrodes, which are parallel to the magnetic field lines in cylindrical configuration and enclose the ICR measurement cell as mantle-like covers, as shown in FIG. 1. Ions introduced into the ICR measurement cell near the axis are brought to orbiting radii by using two of these longitudinal electrodes. During this process, ions of the same mass-to-charge ratio are excited as coherently as possible to obtain a synchronously revolving bundle of ions. The other two electrodes are used to measure the orbiting motion of ions by their image currents induced in the electrodes when the ions fly nearby. One normally speaks of “image currents”, although actually the induced “image voltages” are measured. Filling the ions into the ICR measurement cell, ion excitation and ion detection occur in sequential phases of the operation.
Since the ratio m/z of the mass m to the number z of elementary charges of the ions (called in the following “mass-to-charge ratio” or simply “mass”) is unknown before the measurement, the excitation of the ions occurs by a mixture of excitation frequencies. It can be a mixture in time with temporally increasing or decreasing frequencies (this is called a “chirp”), or it can be a synchronous mixture of all frequencies, calculated by a computer (this is called a “synch pulse”). The synchronous mixture of the frequencies can be configured by a special selection of phases in a way that the amplitudes of the mixture remain within the dynamic range of the digital to analogue converter that is used to generate the temporal progressions of analog voltages for the mixture.
The image currents induced by the ions in the detection electrodes are amplified, digitized and the circular frequencies they contain investigated using Fourier analysis. The initially measured image current values in a “time domain” are transformed using Fourier analysis into a “frequency domain”. Therefore, this type of mass spectrometry is also called the “Fourier transform mass spectrometry” (FTMS). Using the peaks of the signals obtained in the frequency domain, the mass-to-charge ratios of the ions, as well as their intensities are determined subsequently. Due to the extraordinary constancy of the magnetic fields used and due to the high precision of the frequency measurements, an unusually high precision of the mass determination can be achieved. Currently, Fourier transform mass spectrometry is the most precise one of all kinds of mass spectrometry. The precision finally depends on the number of ion circulations which can be covered by the measurement.
The longitudinal electrodes usually form an ICR measurement cell with square or circular cross section. As depicted in FIG. 1, a cylindrical ICR measurement cell usually contains four cylinder mantle segments as longitudinal electrodes. Cylindrical ICR measurement cells are most frequently used, since this represents the most efficient use of the volume in the magnetic field of a circular coil. When tight bundles of ions of one mass closely approach the detection plates, the image currents become more like square waves. The always-observed spread (blurring) of the ion bundle, as well as the selected distance of the ion orbits to the detection electrodes results to a great extent in sine-shaped image current signals for each ion species. Using these signals, orbiting frequencies, and thus, the masses of ions can easily be determined by Fourier analysis.
Since the ions can freely move in the direction of the magnetic field lines, the ions, which after the introduction into the cell possess velocity components in direction of the magnetic field, must be hindered from exiting the cell. Therefore, the ICR measurement cells are equipped at both ends with electrodes, the so called “trapping electrodes”, in order to avoid ion losses. In classical embodiments, these electrodes carry DC voltages, which repel ions in order to keep them in the ICR measurement cell. Very different forms of this pair of trapping electrodes exist. In the simplest case, these are planar electrodes with a central hole. The hole is for the introduction of ions into the ICR measurement cell. In other cases, additional electrodes are placed outside the ICR measurement cell in form of cylinder mantle segments, which are basically the continuation of the internal cylinder mantle segments of the ICR cell and carry the trapping voltages. Hence, an open cylinder is formed without the end walls. These are called “open ICR cells”.
Both inside the open cells and inside the cells with end electrodes, the ion-repelling potentials of the trapping electrodes form a potential well with a parabolic potential profile along the axis of the ICR measurement cell. The potential profile only weakly depends on the shape of the trapping electrodes. The potential profile shows a minimum exactly at the center of the cell, if the repelling potentials are equally high at the trapping electrodes on both sides. Since the ions introduced into the cell have velocities in axial direction, they perform axial oscillations inside this potential well. These movements are called the “trapping oscillations”. The amplitude of these oscillations depends on the kinetic energy of the ions.
Different methods exist for introducing ions into the ICR measurement cell and capturing them inside the cell, e.g. the “sidekick” method or a method with dynamic increase of the potential, which however will not be discussed here in further detail. The person skilled in the art knows these methods.
The electric field outside the axis of the ICR measurement cell is more complicated. Due to the potentials of the trapping electrodes located at both ends, it inevitably contains electrical field components in radial direction, which generate a second kind of motion of ions during the excitation: the magnetron motion. The magnetron motion is a circular motion around the axis of the ICR measurement cell. It is, however, much slower than the cyclotron motion. After a successful cyclotron excitation the magnetron motion remains much smaller than the cyclotron orbits. The magnetron orbiting makes the centers of the of the cyclotron orbits circle around the axis of the ICR measurement cell, so that the ions describe trajectories of a cycloidal motion.
The superposition of the magnetron and cyclotron motions is actually an unwanted appearance, which leads to a shift of the cyclotron frequency. Additionally, it leads to a decrease of the useful volume of the ICR measurement cell. The measured orbiting frequency ω+ (the “reduced cyclotron frequency”) under exclusion of additional space charge effects, that is, for very low numbers of ions in the ICR measurement cell given as
            ω      +        =                            ω          c                2            +                                                  ω              c              2                        4                    -                                    ω              t              2                        2                                ,where ωc is the unperturbed cyclotron frequency and ωt is the frequency of the trapping oscillation. The trapping oscillation determines the influence of the magnetron circulation on the cyclotron motion.
An ICR measurement cell without magnetron circulation would be of great advantage, as the cyclotron frequency could be directly measured and no corrections would need to be undertaken.
In the patent application publication DE 10 2004 038 661 A (J. Franzen and N. Nikolaev) an ICR measurement cell is described, which is enclosed by trapping electrodes in form of radiofrequency grids. This radiofrequency (RF) grid generates an ion-repelling pseudopotential in its very close vicinity, directly before the grid. However, no electric field exists in areas distant from the grid, i.e. in most of the ICR measurement cell. Thus, the cyclotron motion is not perturbed in this cell. During the excitation, a normal trapping DC voltage is connected to the grid. Therefore a magnetron motion appears for a short time. However, after removal of the trapping DC voltage magnetron motion disappears, so that the only orbiting motion that remains is the cyclotron motion, of which the center is now not exactly on the axis of the ICR measurement cell. It is, however, difficult in this ICR measurement cell to perform an unperturbed homogeneous excitation of ions, since the RF voltage used for the excitation of ions generates an electric RF field that is not equal in all cross sections of the ICR measurement cell along its axis. In addition, the RF voltage irradiated by the trapping grid is also received at the detection electrodes, which significantly disturbs the detection of the tiny image currents.
In the patent application publication DE 10 2004 061 821 A1 (J. Franzen and N. Nikolaev) an improved ICR measurement cell is described, in which the trapping electrodes are not driven with radiofrequency voltage. Instead, a grid made of radial spokes is used. The spokes are connected alternately to positive and negative DC voltages. If the ions fly on their cyclotron radii near the spokes, then they fly through the alternating and strongly inhomogeneous positive and negative fields around the spokes. The alternating attraction and repulsion of the ions leads to a flat zigzag orbit. However, during the repelling the ions are always closer to the grid bars than during the attraction. In time average, this leads to a repelling of ions. This repelling can be seen analogous to the repelling of ions from a wire with radiofrequency voltage. In case of structures of electrodes with RF voltage, a repelling “pseudopotential” is generated. In this case of alternating and strongly inhomogeneous DC potentials, the pseudopotential may be called a “motion-induced pseudopotential”. This setup avoids the disturbances of the image current detections by an RF voltage, since only DC voltages are used here. Such a setup to trap ions in an ICR measurement cell with alternately connected DC voltages of different polarity for the generation of the motion induced pseudopotential will be called in the following a “trapping spoke grid”.
Other structures can also be used instead of a spoke grid, e.g. a grid consisting of dot-shaped electrode tips. When the tips are connected alternately to positive and negative voltages, also here, a motion-induced pseudopotential is generated, that repels ions. Such a grid made of electrode tips has slight disadvantages when compared with the grid of radial spokes. Nevertheless, the term “trapping spoke grid” should include a grid made of dot shaped electrode tips.
In the ICR measurement cells with trapping spoke grids, a trapping DC voltage is applied to the spokes or to the tips during the capture of ions and during the excitation to larger cyclotron orbits. Consequently, magnetron motions appear during capture and excitation of ions, which again freeze upon removal of these DC voltages and leave ions on their pure cyclotron orbits with centers slightly off the cell axis.
The homogeneous excitation of ions to larger cyclotron orbits can be improved using a special embodiment of the trapping spoke grid with excitation frequency irradiating electrodes scattered between the spokes, as described in the already mentioned patent DE 39 14 838 C2 (M. Allemann and P. Caravatti) for an “infinity cell”. However, experiments have shown, that although the complex electrode forms needed do reduce the ion losses in the excitation, they do not satisfactorily show the expected effect of ion repelling during orbiting of the ions due to the modified trapping spoke grid. Therefore, there is still a search on how to combine a clean excitation of ions to larger cyclotron orbits with the repulsing effect of the trapping spoke grid.
The vacuum in the ICR measurement cell has to be as good as possible, because during the measurement of the image currents no collisions of ions with the residual gas molecules should take place. Every collision of an ion with a residual gas molecule gets the ions out of the orbiting phase of the remaining ions with the same specific mass. The loss of the phase homogeneity (coherence) leads to a decrease of image currents and to a continuous reduction of the signal-to-noise ratio, which also reduces the usable time of the detection. For high resolution experiments the time of the detection should be at least some hundreds of milliseconds, ideally some seconds. Thus, a vacuum in the range of 10−7 to 10−9 Pascal is required here.
In addition to a bad vacuum, the space charge in the ion cloud extensively influences the measurement. The Coulomb repulsion between the ions of the same polarity and the elastic scattering of the ions traveling with a cloud by the ions in the passed other clouds lead to multiple disturbances. As a result of these disturbances the ion cloud undergoes a radial expansion, it rotates and spreads out. In addition to the effects of pressure, in contemporary instruments, space charge is the most significant limitation to the achievement of a high mass precision. The space charge leads to a shift of the circular frequencies, which cannot be taken into account by just a simple mass calibration. Also, a control of the number of the ions filled into the ICR measurement cell only helps under certain conditions. The experience always shows that it is not only the number of ions within the ICR measurement cell which influences the shift of the frequencies, but it is also the distribution of the charges over different masses and different charge state of ions. Thus, the shift of the orbiting frequencies does not only depend on the total intensity of the space charge, but also on the composition of the ion mixture.
In the patent application DE 10 2007 047 075.6 (G. Baykut and R. Jertz) a method of operating an ICR measurement cell is described, where the orbiting frequencies become widely independent of the space charge. By applying here a slightly attractive net potential, the ions are pulled closer to the trapping spoke grid. In this method of operation the space charge in the cell can be changed by a factor of hundred without causing a change in the measured orbiting frequency. If a mass calibration is performed in this state of the operation, it would remain valid throughout the following measurements independent of the amount of ions filled into the ICR measurement cell. The reason for this behavior is not yet known.
The image currents of the circulating ions need not necessarily be measured in the longitudinal electrodes of the ICR measurement cell. In adequately shaped cells, ions can also be measured in the end electrodes, as described in the patent application DE 10 2007 017 053.1 (R. Zubarev and A. Misharin). The end electrodes have to be divided in radial segments. This way, some elements carry the trapping voltage and other elements are used for the detection of the image currents.
The detection of the tiny image currents is a challenge for the electrical connections between the detection electrodes and the amplifiers. The conductors must be of extremely low impedance, and should not contain any contacts, of which the contact voltages are temperature dependent. Circuit switches without sufficiently low impedance contacts or those with vibration-dependent resistances are not allowed. Therefore, the detection electrodes cannot be used for other purposes by switching between detection and supplying other voltages. It is proven to be the best, if the detection electrodes are firmly contacted to the amplifier by low impedance solid wires made of silver.